Apparatuses and methods of reprogramming memory cells

ABSTRACT

Apparatuses and methods for reprogramming memory cells are described. One or more methods for memory cell operation includes programming a number of memory cells such that each of the number of memory cells are at either a first program state or a second program state, the second program state having a first program verify voltage associated therewith; and reprogramming the number of memory cells such that at least one of the number of memory cells is reprogrammed to a third program state having a second program verify voltage associated therewith, wherein those of the number of memory cells having a threshold voltage less than the second program verify voltage represent a same data value.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor memory devicesand methods, and more particularly, to methods and devices for memorycell operation.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits and/or external removable devices in computers orother electronic devices. There are many different types of memoryincluding random-access memory (RAM), read only memory (ROM), dynamicrandom access memory (DRAM), synchronous dynamic random access memory(SDRAM), phase change random access memory (PCRAM), and flash memory,among others.

Flash memory devices can be utilized as non-volatile memory for a widerange of electronic applications. Flash memory devices typically use aone-transistor memory cell that allows for high memory densities, highreliability, and low power consumption. Uses for flash memory includememory for solid state drives (SSDs), personal computers, personaldigital assistants (PDAs), digital cameras, cellular telephones,portable music players, e.g., MP3 players, and movie players, amongother electronic devices.

Two common types of flash memory array architectures are the “NAND” and“NOR” architectures, so called for the logical form in which the basicmemory cell configuration of each is arranged. A NAND array architecturearranges its array of memory cells in a matrix such that the controlgates of each memory cell in a “row” of the array are coupled to (and insome cases form) an access line, which is commonly referred to in theart as a “word line”. However each memory cell is not directly coupledto a data line (which is commonly referred to as a bit line, in the art)by its drain. Instead, the memory cells of the array are coupledtogether in series, source to drain, between a common source and a dataline, where the memory cells commonly coupled to a particular data lineare referred to as a “column”.

Memory cells in a NAND array architecture can be programmed to atargeted, e.g., desired, program state. For example, electric charge canbe placed on or removed from a charge storage structure, e.g., afloating gate or charge trap, of a memory cell to put the cell into oneof a number of program states. For example, a single level cell (SLC)can be programmed to one of two program states, such as to represent abinary data value, e.g., “1” or “0”, stored by the cell. In a NANDarchitecture, a memory cell is erased prior to being programmed, and thecell is not reprogrammed prior to being erased. The memory cells of aNAND array are erased together in groups referred to as “blocks”. Eachblock can include a number of pages of memory cells that are programmedand/or read together as a group. As such, pages of memory cells of aparticular block are not reprogrammed individually without first erasingthe entire block. Requiring a block to be erased before cells of theblock can be reprogrammed can increase the wear of a memory device,among other drawbacks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a portion of an apparatus in the form of anon-volatile memory array in accordance with one or more embodiments ofthe present disclosure.

FIGS. 2A-2C are diagrams illustrating a number of threshold voltagedistributions corresponding to program states associated with memorycells programmed and/or reprogrammed in accordance with one or moreembodiments of the present disclosure.

FIG. 3 is a functional block diagram of an apparatus in the form of acomputing system including at least one memory system in accordance withone or more embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure includes apparatuses and methods of reprogrammingmemory cells. One or more methods for memory cell operation includesprogramming a number of memory cells such that each of the number ofmemory cells are at either a first program state or a second programstate, the second program state having a first program verify voltageassociated therewith; and reprogramming the number of memory cells suchthat at least one of the number of memory cells is reprogrammed to athird program state having a second program verify voltage associatedtherewith, wherein those of the number of memory cells having athreshold voltage less than the second program verify voltage representa same data value.

In the following detailed description of the present disclosure,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration how one or more embodimentsof the disclosure may be practiced. These embodiments are described insufficient detail to enable those of ordinary skill in the art topractice the embodiments of this disclosure, and it is to be understoodthat other embodiments may be utilized and that process, electrical,and/or structural changes may be made without departing from the scopeof the present disclosure.

As used herein, the designators “N,” “M,” and “R”, particularly withrespect to reference numerals in the drawings, indicates that a numberof the particular feature so designated can be included with one or moreembodiments of the present disclosure. Additionally, as used herein, “anumber of” something can refer to one or more such things. For example,a number of memory devices can refer to one or more memory devices.

The figures herein follow a numbering convention in which the firstdigit or digits correspond to the drawing figure number and theremaining digits identify an element or component in the drawing.Similar elements or components between different figures may beidentified by the use of similar digits. For example, 110 may referenceelement “10” in FIG. 1, and a similar element may be referenced as 210in FIG. 2. As will be appreciated, elements shown in the variousembodiments herein can be added, exchanged, and/or eliminated so as toprovide a number of additional embodiments of the present disclosure. Inaddition, as will be appreciated, the proportion and the relative scaleof the elements provided in the figures are intended to illustrate theembodiments of the present invention, and should not be taken in alimiting sense.

FIG. 1 is a schematic of a portion of an apparatus in the form of anon-volatile memory array 100 in accordance with one or more embodimentsof the present disclosure. The embodiment of FIG. 1 illustrates a NANDarchitecture non-volatile memory array. However, embodiments describedherein are not limited to this example. As shown in FIG. 1, the memoryarray 100 includes access lines, e.g., word lines 105-1, . . . , 105-Nand intersecting data lines, e.g., local bit lines 107-1, 107-2, 107-3,. . . , 107-M. For ease of addressing in the digital environment, thenumber of word lines 105-1, . . . , 105-N and the number of local bitlines 107-1, 107-2, 107-3, . . . , 107-M can be some power of two, e.g.,256 word lines by 4,096 bit lines.

Memory array 100 includes NAND strings 109-1, 109-2, 109-3, . . . ,109-M. Each NAND string includes non-volatile memory cells 111-1, . . ., 111-N, each communicatively coupled to a respective word line 105-1, .. . , 105-N. Each NAND string (and its constituent memory cells) is alsoassociated with a local bit line 107-1, 107-2, 107-3, . . . , 107-M. Thememory cells 111-1, . . . , 111-N of each NAND string 109-1, 109-2,109-3, . . . , 109-M are connected in series source to drain between asource select gate (SGS), e.g., a field-effect transistor (FET) 113, anda drain select gate (SGD), e.g., FET 119. Each source select gate 113 isconfigured to selectively couple a respective NAND string to a commonsource 123 responsive to a signal on source select line 117, while eachdrain select gate 119 is configured to selectively couple a respectiveNAND string to a respective bit line responsive to a signal on drainselect line 115.

As shown in the embodiment illustrated in FIG. 1, a source of sourceselect gate 113 is connected to a common source line 123. The drain ofsource select gate 113 is connected to the source of the memory cell111-1 of the corresponding NAND string 109-1. The drain of drain selectgate 119 is connected to bit line 107-1 of the corresponding NAND string109-1 at drain contact 121-1. The source of drain select gate 119 isconnected to the drain of the last memory cell 111-N, e.g., afloating-gate transistor, of the corresponding NAND string 109-1. In oneor more embodiments, construction of the non-volatile memory cells 1114,. . . , 111-N includes a source, a drain, a floating gate or othercharge storage structure, and a control gate. The memory cells 111-1, .. . , 111-N have their control gates coupled to a word line, 105-1, . .. , 105-N respectively. A NOR array architecture would be similarly laidout, except that the string of memory cells would be coupled in parallelbetween the select gates.

As one of ordinary skill in the art will appreciate, subsets of cellscoupled to a selected word line, e.g., 105-1, . . . , 105-N, can beprogrammed and/or sensed, e.g., read, together as a group. A number ofcells programmed and/or sensed together can correspond to a page ofdata. In association with a sensing operation, a number of cells coupledto a particular word line and programmed together to respective programstates can be referred to as a target page. A programming operation,e.g., a write operation, can include applying a number of programpulses, e.g., 16V-20V, to a selected word line in order to increase thethreshold voltage (Vt) of selected cells coupled to that selected accessline to a desired program voltage level corresponding to a targetedprogram state.

A sensing operation, such as a read or program verify operation, caninclude sensing a voltage and/or current change of a bit line coupled toa selected cell in order to determine the program state of the selectedcell. The sensing operation can include precharging a bit line andsensing the discharge when a selected cell begins to conduct.

Sensing the program state of a selected cell can include providing anumber of sensing voltages, e.g., read voltages, to a selected word linewhile providing a number of voltages, e.g., read pass voltages, to theword lines coupled to the unselected cells of the string sufficient toplace the unselected cells in a conducting state independent of thethreshold voltage of the unselected cells. The bit line corresponding tothe selected cell being read and/or verified can be sensed to determinewhether or not the selected cell conducts in response to the particularsensing voltage applied to the selected word line. For example, theprogram state of a selected cell can be determined by the word linevoltage at which the bit line current reaches a particular referencecurrent associated with a particular state.

As one of ordinary skill in the art will appreciate, in a sensingoperation performed on a selected memory cell in a NAND string, theunselected memory cells of the string are biased so as to be in aconducting state. In such a sensing operation, the program state of theselected cell can be determined based on the current and/or voltagesensed on the bit line corresponding to the string. For instance, theprogram state of the selected cell can be determined based on whetherthe bit line current changes by a particular amount or reaches aparticular level in a given time period.

As an example, the memory cells of an array such as array 100 can besingle level cells (SLCs). SLCs can be single-bit, e.g., two-state,memory cells. That is, the cells can be programmed to one of two programstates, e.g., L0 and L1, respectively. In operation, a number of memorycells, such as in a selected block, can be programmed such that theyhave a Vt level corresponding to either L0 or L1. As an example, programstate L0 can represent a stored data value such as binary “1”. Programstate L1 can represent a stored data value such as binary “0”. However,embodiments are not limited to these data assignments, e.g., programstate L0 can represent binary “0” and program state L1 can representbinary “1”.

As further described below, in one or more embodiments, groups of memorycells, such as a number of memory cells on a wordline or in a block, forexample, can be reprogrammed without being erased. For example, thememory cells can be reprogrammed, without erasing the memory cells priorto reprogramming, so that a portion of the memory cells are reprogrammedto a new program state representing a data value, such as binary “0”,and the remaining memory cells remain in their originally programmedprogram states, where those original program states represent anotherdata value, such as binary “1”. At least a portion of the memory cellsthat are not reprogrammed to the new program state may have stored thedata value represented by the new program state after reprogramming,e.g., binary “0”, prior to the reprogramming operation. According tosuch an example, after the reprogramming operation, the memory cellsthat previously stored a binary “1” or “0” and have not had theirthreshold voltage increased to the new program state now store, forexample, a binary “1”, and memory cells that have their thresholdvoltage increased to the new program state now store, for example, abinary “0”.

FIGS. 2A-2C are diagrams illustrating a number of threshold voltagedistributions corresponding to program states associated with memorycells programmed and/or reprogrammed in accordance with one or moreembodiments of the present disclosure. The memory cells associated withFIGS. 2A-2C can be memory cells such as cells 111-1, . . . , 111-Ndescribed in connection with FIG. 1. The memory cells associated withFIGS. 2A-2C are one-bit, e.g., two-state, SLCs. However, embodiments ofthe present disclosure are not limited to this example.

The Vt distributions 220 and 222 of FIG. 2A correspond to a number ofmemory cells programmed to one of two program states, e.g., L0 or L1.The Vt distributions 224-1, 224-2, and 226 of FIG. 2B correspond to thenumber of memory cells associated with FIG. 2A after to a reprogrammingoperation in which the cells are reprogrammed from states L0 and L1 toone of program states L0′ and L1′. The Vt distributions 228-1, 228-2,228-3, and 230 of FIG. 2C correspond to the number of memory cellsassociated with FIGS. 2A and 2B after a subsequent reprogrammingoperation in which the cells are reprogrammed from states L0′ and L1′ toone of program states L0″ and L1″.

In FIG. 2A, threshold voltage distribution 220 corresponds to a numberof memory cells that are programmed to a first program state, e.g., L0.In at least some embodiments, programming a memory cells to the firstprogram state may simply include allowing a memory cell to remain in anerased state. In other words, the first program state may actually bethe erased state. Threshold voltage distribution 222 corresponds to anumber of memory cells that are programmed from an erased state, whichcould be the same as the first program state, to a second program state,e.g., L1. A first program verify voltage 223 (V_(PV1)) is used whenprogramming the number of memory cells to verify that the memory cellsto be programmed to state L1 have a threshold voltage higher thanV_(PV1) 223. Threshold voltage distributions 220 and 222 can correspondto a group of cells, e.g., a page or a block, programmed as part of afirst program operation. In this example, the memory cells programmed tothe program state L0 represent, e.g., store, a first data value, e.g.,binary “1”. The memory cells programmed to the program state L1represent a second data value, e.g., binary “0”.

The threshold voltage distributions 224-1, 224-2, and 226 shown in FIG.2B correspond to the number of memory cells associated with FIG. 2Aafter a reprogramming operation, which in some example, may be, but isnot required to be, a first of more than one reprogramming operations.The reprogramming operation includes programming the memory cellspreviously programmed to one of program states L0 or L1 to one ofprogram states L0′ or L1′. For instance, Vt distribution 226 correspondsto those cells previously programmed to program state L0 or L1 and whichare subsequently reprogrammed to the L1 program state. Vt distributions224-1 and 224-2 correspond to those cells previously programmed toprogram state L0 or program state L1 and which are not programmed toprogram state L1′ during the reprogramming operation. After thereprogramming operation, original program states L0 and L1 may becollectively considered program state L0′. A second program verifyvoltage 227 (V_(PV2)) is used when reprogramming the number of memorycells to verify that the memory cells to be programmed to the programstate L1′ have a threshold voltage higher than V_(PV2) 227. Thereprogramming operation associated with FIG. 2B includes reprogrammingthe number of memory cells associated with FIG. 2A without erasing thenumber of memory cells. After the reprogramming operation associatedwith FIG. 2B, those memory cells corresponding to Vt distributions 224-1and 224-2 represent a same data value, e.g. binary “1” and those memorycells corresponding to Vt distribution 226 represent binary “0”. Assuch, one or more memory cells within distribution 224-2, which stored abinary “0” prior to reprogramming, e.g., cells previously programmed tostate L1, store binary “1” subsequent to reprogramming.

The threshold voltage distributions 228-1, 228-2, 228-3, and 230 shownin FIG. 2C correspond to the number of memory cells associated withFIGS. 2A and 2B after a second reprogramming operation, however a secondreprogramming operation need not be part of all embodiments of thepresent disclosure. In other words, some embodiments may onlyincorporate a single reprogramming operation. The reprogrammingoperation associated with FIG. 2C includes programming the memory cellspreviously programmed to one of program states L0′ or L1′ to one ofprogram states L0″ or L1″. For instance, Vt distribution 230 correspondsto those cells previously programmed to program state L0′ or L1′ andwhich are subsequently reprogrammed to the L1″ program state. Vtdistributions 228-1, 228-2, and 228-3 correspond to those cellspreviously programmed to program state L0′ or program state L1′ andwhich are not reprogrammed to program state L1″ during the secondreprogramming operation. A third program verify voltage 231 (V_(PV3)) isused when reprogramming the number of memory cells to verify that thememory cells to be programmed to the program state L1″ have a thresholdvoltage higher than V_(PV3) 231. The reprogramming operation associatedwith FIG. 2C includes reprogramming the number of memory cellsassociated with FIGS. 2A and 2B without erasing the number of memorycells. After the reprogramming operation associated with FIG. 2C, thosememory cells corresponding to Vt distributions 228-1, 228-2, and 228-3represent a same data value, e.g. binary “1” and those memory cellscorresponding to Vt distribution 230 represent a different data value,e.g., a binary “0”. As such, one or more memory cells withindistribution 228-3 which stored a binary “0” prior to the secondreprogramming operation, e.g., cells previously programmed to state L1′,store a binary “1” subsequent to the second reprogramming operation.

In one or more embodiments, the number of memory cells can bereprogrammed a number of times. The number of times the number of memorycells can be reprogrammed can be limited by the available programmingwindow, e.g., voltage range. Each time a memory cell and/or a group ofmemory cells, such as a page, a block, and/or memory cells on awordline, for example, are programmed and/or reprogrammed an indication,e.g., a flag, can be stored. The flag can indicate whether and/or howmany times a particular memory cell and/or group of memory cells, e.g.,page, have been reprogrammed. The indication can be stored in one ormore memory cells and/or in a register, for example. In one or moreembodiments, the flag can provide an indication on a wordline bywordline and/or a block by block basis that memory cells of a particularwordline and/or block have been programmed and/or reprogrammed.

FIG. 3 is a functional block diagram of an apparatus in the form of acomputing system 340 including at least one memory system 344 inaccordance with one or more embodiments of the present disclosure.Memory system 344 can be, for example, a solid state drive (SSD). In theembodiment illustrated in FIG. 1, memory system 344 includes a physicalhost interface 346, a number of memory devices 350-1, . . . , 350-R(e.g., solid state memory devices), and a controller 348 (e.g., an SSDcontroller) coupled to physical host interface 346 and memory devices310-1, . . . , 310-N.

Physical host interface 346 can be used to communicate informationbetween memory system 344 and another device such as a host 342. Host342 can include a memory access device (e.g., a processor). One ofordinary skill in the art will appreciate that “a processor” can intenda number of processors, such as a parallel processing system, a numberof coprocessors, etc. Example hosts can include laptop computers,personal computers, digital cameras, digital recording and playbackdevices, mobile telephones, PDAs, memory card readers, interface hubs,and the like.

Physical host interface 346 can be in the form of a standardizedphysical interface. For example, when memory system 344 is used forinformation storage in computing system 340, physical host interface 346can be a serial advanced technology attachment (SATA) physicalinterface, a peripheral component interconnect express (PCIe) physicalinterface, or a universal serial bus (USB) physical interface, amongother physical connectors and/or interfaces. In general, however,physical host interface 346 can provide an interface for passingcontrol, address, information (e.g., data), and other signals betweenmemory system 344 and a host (e.g., host 342) having compatiblereceptors for physical host interface 346.

Controller 348 can include, for example, control circuitry and/orfirmware. Controller 348 can be included on the same physical device(e.g., the same die) as memory devices 350-1, . . . , 350-R. Forexample, controller 348 can be an application specific integratedcircuit (ASIC) coupled to a printed circuit board including physicalhost interface 346 and memory devices 350-1, . . . , 350-R.Alternatively, controller 348 can be included on a separate physicaldevice that is communicatively coupled to the physical device thatincludes memory devices 350-1, . . . , 350-R.

Controller 348 can communicate with memory devices 350-1, . . . , 350-Rto sense (e.g., read), program (e.g., write), and/or erase information,among other operations. Controller 348 can have circuitry that may be anumber of integrated circuits and/or discrete components. In a number ofembodiments, the circuitry in controller 348 may include controlcircuitry for controlling access across memory devices 350-1, . . . ,350-R and/or circuitry for providing a translation layer between host342 and memory system 344. The controller 348 includes sense circuitry352 to determine the program state of memory cells, for instance.

Memory devices 350-1, . . . , 350-R can include, for example, a numberof non-volatile memory arrays (e.g., arrays of non-volatile memorycells). For instance, memory devices 350-1, . . . , 350-R can includearrays of memory cells, such as array 100 described in FIG. 1, which canbe operated in accordance with embodiments described herein. As will beappreciated by those of ordinary skill in the art, the memory cells inthe memory arrays of memory devices 350-1, . . . , 350-R can be in aNAND architecture, a NOR architecture, or some other memory arrayarchitecture.

The memory arrays of memory devices 350-1, . . . , 350-R can include anumber of memory cells that can be grouped. As used herein, a group caninclude a number of memory cells, such as a page, block, plane, die, anentire array, or other groups of memory cells. For example, some memoryarrays can include a number of pages of memory cells that make up ablock of memory cells. A number of blocks can be included in a plane ofmemory cells. A number of planes of memory cells can be included on adie. As an example, a 128 GB memory device can include 4320 bytes ofinformation per page, 128 pages per block, 2048 blocks per plane, and 16planes per device.

The embodiment illustrated in FIG. 3 can include additional circuitrythat is not illustrated so as not to obscure embodiments of the presentdisclosure. For example, memory device 303 can include address circuitryto latch address signals provided over I/O connectors through I/Ocircuitry. Address signals can be received and decoded by a row decoderand a column decoder, to access memory array 300. It will be appreciatedby those skilled in the art that the number of address input connectorscan depend on the density and architecture of memory device 303 and/ormemory array 300.

CONCLUSION

The present disclosure includes apparatuses and methods forreprogramming memory cells. One or more methods for memory celloperation includes programming a number of memory cells such that eachof the number of memory cells are at either a first program state or asecond program state, the second program state having a first programverify voltage associated therewith; and reprogramming the number ofmemory cells such that at least one of the number of memory cells isreprogrammed to a third program state having a second program verifyvoltage associated therewith, wherein those of the number of memorycells having a threshold voltage less than the second program verifyvoltage represent a same data value.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of one or more embodiments of the presentdisclosure. It is to be understood that the above description has beenmade in an illustrative fashion, and not a restrictive one. Combinationof the above embodiments, and other embodiments not specificallydescribed herein will be apparent to those of skill in the art uponreviewing the above description. The scope of the one or moreembodiments of the present disclosure includes other applications inwhich the above structures and methods are used. Therefore, the scope ofone or more embodiments of the present disclosure should be determinedwith reference to the appended claims, along with the full range ofequivalents to which such claims are entitled.

In the foregoing Detailed Description, some features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

1. A method for operating memory cells, comprising: programming a numberof memory cells such that each of the number of memory cells are ateither a first program state or a second program state, the secondprogram state having a first program verify voltage associatedtherewith; and reprogramming the number of memory cells such that atleast one of the number of memory cells is reprogrammed to a thirdprogram state having a second program verify voltage associatedtherewith, wherein those of the number of memory cells having athreshold voltage less than the second program verify voltage representa same data value.
 2. The method of claim 1, wherein programming thenumber of memory cells includes programming a number of single levelcells (SLC) in a NAND array.
 3. The method of claim 1, includingproviding an indication on an access line by access line basis thatmemory cells of a particular access line have been programmed.
 4. Themethod of claim 1, including providing an indication on an access lineby access line basis that memory cells of a particular access line havebeen reprogrammed.
 5. The method of claim 1, including providing anindication on a block by block basis that memory cells of a particularblock have been reprogrammed.
 6. The method of claim 1, wherein afterreprogramming the number of memory cells, the memory cells of the numberof memory cells having a threshold voltage greater than the secondprogram verify voltage represent a data value that is different than thedata value represented by the memory cells of the number of memory cellshaving a threshold voltage less than the second program verify voltage.7. A method for operating an array of memory cells, comprising:programming a block of memory cells such that each of a number of memorycells in the block are at either a first program state or a secondprogram state, the second program state having a first program verifyvoltage associated therewith; and reprogramming at least one of thenumber of memory cells from its respective first or second program stateto a third program state having a second program verify voltageassociated therewith, wherein the second program verify voltage isgreater than threshold voltages of memory cells at the first and secondprogram states and wherein those of the number of memory cells thatremain at the second program state after reprogramming represent adifferent data value than prior to reprogramming.
 8. The method of claim7, wherein, prior to reprogramming the at least one of the number ofmemory cells from its respective program state to the third programstate, those memory cells at the first program state represent adifferent data value than those memory cells at the second data state.9. The method of claim 7, wherein, after reprogramming the at least oneof the number of memory cells from its respective program state to thethird program state, those memory cells at the first and second programstates represent a same data value.
 10. The method of claim 7, wherein,after reprogramming the at least one of the number of memory cells fromits respective program state to the third program state, those memorycells at the third program state represent a data value that isdifferent than a data value represented by memory cells at the first andsecond program states.
 11. The method of claim 7, includingreprogramming at least one of the number of memory cells from itsrespective program state to a fourth program state having a thirdprogram verify voltage associated therewith, wherein the third programverify voltage is greater than threshold voltages of memory cells at thefirst, second, and third program states.
 12. The method of claim 11,wherein, prior to reprogramming the least one of the number of memorycells from its respective program state to the fourth program state,those memory cells at the third program state represent a different datavalue than those memory cells at the first and second program states.13. The method of claim 11, wherein, after reprogramming the least oneof the number of memory cells from its respective program state to thefourth program state, those memory cells at the first, second, and thirdprogram states represent a same data value.
 14. The method of claim 11,wherein, after reprogramming the least one of the number of memory cellsfrom its respective program state to the fourth program state, thosememory cells at the fourth program state represent a data value that isdifferent than a data value represented by memory cells at the first,second, and third program states.
 15. The method of claim 7, includingindicating that the memory cells have been reprogrammed.
 16. A methodfor operating memory cells, comprising: adjusting a program verifyvoltage from an initial voltage to an adjusted program verify voltagethat is greater than a threshold voltage of a group of memory cells eachprogrammed to either a first or second program state; and reprogrammingthe group such that a number of memory cells of the group arereprogrammed to a third program state associated with the adjustedprogram verify voltage, wherein those memory cells having a thresholdvoltage less than the adjusted program verify voltage represent a samedata value.
 17. The method of claim 16, wherein, after reprogramming,the number of memory cells at the third program state represent a datavalue that is different than a data value represented by memory cells atthe first and second program states.
 18. The method of claim 16,including providing a flag that indicates the group of memory cells havebeen reprogrammed.
 19. The method of claim 16, wherein the number ofmemory cells comprises a first number of memory cells, and includingreprogramming the group of memory cells such that a second number ofmemory cells of the group are reprogrammed from their respective first,second, or third program state to a fourth program state associated withan adjusted program verify voltage that is greater than program verifyvoltage associated with the first, second, or third program states. 20.An apparatus, comprising: an array of memory cells; and a controlleroperably coupled to the array and configured to: program each memorycell of a group of memory cells to either a first program state or asecond program state; and reprogram the memory cells of the group suchthat a number of memory cells of the group are reprogrammed from arespective one of the first and second program states to a third programstate, wherein prior to reprogramming, the first and second programstates represent different data values and after reprogramming the firstand second program states represent a same data value.
 21. The apparatusof claim 20, wherein, prior to reprogramming, the first program staterepresents a first data value and the second program state represents asecond data value; and wherein, after reprogramming, the third programstate represents either the first data value or the second data value.22. The apparatus of claim 20, wherein, the controller is configured toreprogram the memory cells of the group without first erasing the groupof memory cells.
 23. The apparatus of claim 20, wherein the controlleris configured to reprogram the memory cells of the group on an accessline by access line basis.
 24. The apparatus of claim 20, wherein thegroup of memory cells includes only single level cells.
 25. Anapparatus, comprising: an array of memory cells; and a controlleroperably coupled to the array and configured to: adjust a program verifyvoltage from an initial voltage to an adjusted program verify voltagethat is greater than a threshold voltage of a group of memory cells,wherein each of the memory cells of the group is programmed to either afirst or second program state, and wherein the memory cells of the groupare reprogrammed such that a number of memory cells of the group arereprogrammed to a third program state associated with the adjustedprogram verify voltage.
 26. The apparatus of claim 25, wherein thecontroller is configured to reprogram the memory cells of the group suchthat a number of memory cells of the group reprogrammed to the first orsecond program state are considered erased.
 27. The apparatus of claim26, wherein the controller is configured to provide an indication thatthe group has been reprogrammed.
 28. The apparatus of claim 27, whereinthe indication is a flag.